I am able to provide an opinion on the most likely author (among a pool of suspects) of anonymous letters or threatening mail. The main question that has to be answered is “who wrote it?”.  Categories of anonymous handwriting include:

• Anonymous messages that are criminal in nature. These would include handwritten messages containing any kind of threatening content: death threats, threats to reputation or bomb threats. Also, ransom notes, letters of extortion or any written material that forms part of the chain of evidence in a criminal investigation.
• Vicious, persistent and disturbing letters or notes. In these cases a victim becomes the target of a continuous campaign of harassment in the form of hate mail. Very often such letters are in disguised handwriting. Unpleasant sexual innuendo is not unusual in these cases.
• Mischievous notes and drawings created by children and usually disseminated at school, either to fellow pupils, or to teachers. Anonymous letters of complaint against authority figures also fall into this category.
• Graffiti. Messages written or spray-painted onto walls or fixed surfaces.  (Not the same as gangster graffiti and street art.)